1. Field of the Invention
The present invention relates to a system for detecting a position of a reference track, for example, the outermost track on a disc for a disc storage unit in which plural tracks are provided on the disc surface like in the case of a fixed disc storage unit, and in which servo information for detecting an off-track amount of a position of a read/write head relative to the track is written on the disc surface.
2. Description of the Prior Art
In a disc storage unit, the above-mentioned outermost track is usually referred to as a reference track of track number 0, and the serial numbers 1, 2, --- are assigned as track numbers to all other tracks on the disc in the inward sequence of tracks from the outermost track. When the head is moved to a desired track, the number of this track is designated, or alternatively, the difference between the present track number and the desired track number is designated. In case the stored number of a track where the head is currently located has been lost for some reason or other, the head is returned to the position of the reference track to confirm the position of the reference track, so that the number of the track where the head is located is stored correctly again.
When operation of the disc storage unit is stopped, if the head remains located on a track, the head may damage the storage medium on the disc surface when unexpected vibration or shock is applied to the head. When, therefore, the operation is stopped, it is usual that the head is rested in a so-called shipping zone formed on the inner diameter side of the disc separately from the tracks. When the operation of the disc storage unit is resumed, the head is moved from this shipping zone to a track. At this time, if the position of the innermost track on the disc surface can be detected, then this innermost track may be used as a second reference track, in addition to the above-mentioned outermost track, to determine the number of the track where the head is currently located with reference to the second reference track.
In this manner, it is necessary to detect a specific track defined by some standard on the disc surface, that is, a reference track in the present invention, in order to position the head. The reference track, however, does not have particular characteristics different from an ordinary track other than its name, and accordingly it is not possible hitherto to detect this track by discriminating its own characteristics. So far, the position of the reference track is detected by a position detector provided on a head drive system, for instance, a head carriage, a head arm, or a head drive motor. The detector functions like a limit switch, and can be made of a sensor utilizing the interruption of light between a light emitting diode and a phototransistor like a publicly known photo-interrupter, or of a position sensor represented by a Hall element.
Any of these publicly known detection means needs some sensor such as mentioned above, and accordingly it is also necessary to provide a power feed circuit to the sensor or a processing circuit for processing a sensor output. Also, in order to fit a recent demand of a small diameter disc of, for example, about 3.5 inches, with an increased storage capacity, at least hundreds of tracks are formed on the disc surface at a pitch of less than 10 .mu.m. In this case, it is necessary to mount the sensor with an accuracy of 10 .mu.m or less. Therefore, sensor mounting and position adjustment of the mounted sensor are so troublesome that mass production of disc storage units will have to be time-consuming. Moreover, it is usually difficult to obtain accurately a detection output from a sensor with respect to only one reference track, and the sensor output is likely to be obtained from the reference track and several tracks adjacent to the reference track. Consequently, it is also necessary to provide means for picking one particular reference track out of those several tracks. As a result, an arrangement of means for processing a sensor output will become complicated.